Catheter based devices are employed in various medical and surgical applications because they are relatively non-invasive and allow for precise treatment of localized tissues that are otherwise inaccessible. Catheters may be easily inserted and navigated through the blood vessels and arteries, allowing non-invasive access to areas of the body with relatively little trauma. Recently, catheter-based systems have been developed for implementation in tissue ablation for treatment of cardiac arrhythmias such as atrial fibrillation, supra ventricular tachycardia, atrial tachycardia, ventricular tachycardia, ventricular fibrillation, and the like. One such implementation involves the use of fluids with low operating temperatures, or cryogens, to selectively freeze, or “cold-treat”, targeted tissues within the body.
The cryogenic treatment involves cooling a portion of the catheter to a very low temperature through the use of the cryogenic fluid flowing through the catheter. A cryogenic device uses the energy transfer derived from thermodynamic changes occurring in the flow of a cryogen therethrough to create a net transfer of heat flow from the target tissue to the device, through conductive and convective heat transfer between the cryogen and target tissue.
Structurally, cooling can be achieved through injection of high-pressure coolant into a lumen of the catheter. Upon injection, the refrigerant undergoes two primary thermodynamic changes: (i) expanding to low pressure and temperature through positive Joule-Thomson throttling, and (ii) undergoing a phase change from liquid to vapor, thereby absorbing heat of vaporization. The resultant flow of low temperature refrigerant through the device acts to absorb heat from the target tissue and thereby cool the tissue to the desired temperature.
Once refrigerant is injected into the lumen, it may be expanded inside of an expandable element (chamber), which may be positioned proximal to the target tissue. In embodiments, the expandable element may also be thermally conductive. Devices with an expandable element, such as a balloon, may be employed. In such devices, refrigerant is supplied through a catheter lumen into an expandable balloon coupled to such catheter, wherein the refrigerant acts to both: (i) expand the balloon near the target tissue for the purpose of positioning the balloon, and (ii) cool the target tissue proximal to the balloon to cold-treat adjacent tissue.
The expandable element may also serve a second function; blocking the flow of blood through the desired treatment site (occlusion). The catheter is typically of a relatively small diameter and long body, generally determined, by the diameter and length of the vascular pathways leading to the ablation site. The coolant in the catheter is highly susceptible to conductive warming effects due to the relative proximity of the catheter (and coolant) to the body tissue and blood. Furthermore, the rate of cooling is limited by the ability to circulate a sufficient mass flow of coolant through the catheter. Yet there is a requirement that the coolant itself be at a sufficiently low temperature, in some cases below freezing, at the location of the ablation. In addition, while this may be generally true, the occlusion may help reduce the heat-load at the target ablation site and not on the whole catheter.
Blocking the flow of blood using the expandable element allows more effective cooling which facilitates the treatment process and may reduce the treatment period. Effective contact to achieve occlusion may require moving, positioning, anchoring and other mechanisms for locating and stabilizing the conformation of the expandable element of the catheter. Moreover, slight changes in orientation may greatly alter the characteristics of the catheter, so that even when the changes are predictable or measurable, it may become necessary to provide positioning mechanisms of high stability or accuracy to assure adequate treatment at the designated sites. Furthermore, one must assure that the ablation is effective at the target tissue.
Known techniques for visualizing the contact between the expandable element and the target tissue include the use of radiographically opaque contrast medium to enable radiographic-mapping of the target tissue during application and operation of the catheter. Such an imaging technique may not be desirable due to the use of contrast medium and its interaction with the patient tissue. Additionally, it may be desirable to eliminate or minimize the exposure of both patient and clinician to the radiographic-mapping waves used for imaging.
It is desirable therefore, to provide improved catheter systems that are capable of providing an indication of occlusion while eliminating or significantly reducing exposure of the patient and clinician to imaging waves.